Conservative Analytical Assessments of Localized RF Exposure From Small Magnetic Field Sources

A conservative analytical framework for assessing radiofrequency exposure from small magnetic field sources near the body is presented, with an emphasis on the 3 kHz–10 MHz frequency range. Worst-case exposure models are proposed and analyzed for both homogeneous and heterogeneous tissue based on source dimensions, drive current, and separation distance. Electromagnetic analysis of induced field enhancements due to tissue heterogeneity is presented. Maximum drive currents for compliance with the basic restrictions are obtained for both tissue models. In the heterogeneous case, field enhancement in thin regions of low conductivity leads to significantly lower allowable drive levels for nerve stimulation compliance. Guidance is provided regarding how to account for such enhancements for various internal E-field calculation methods. The impact of these field enhancements on 10-g specific absorption rate (SAR) is found to be small, i.e., the homogeneous tissue assumption appears to be sufficiently conservative. A small enhancement factor may be appropriate for 1-g SAR. The benefit of assessing against the basic restrictions instead of the reference levels is also explored. This work can be leveraged by regulatory and standardization bodies to develop exemption levels for small magnetic field sources, e.g., inductive chargers, to significantly reduce compliance burdens.